The T- and B-cell surface marker assay uses monoclonal antibodies to identify the various membrane antigens, and flow cytometry to enumerate the number of cells expressing these differentiation antigens. CD14 is used to exclude monocytes, thereby improving accuracy and enhancing the purity of the lymphocyte population. The results are reported as the percent of lymphocytes that are T cells (CD3+), T-helper (CD3+, CD4+), T-suppressor(CD3+,CD8+), natural killer (CD16+CD56+, CD3-), and B lymphocytes(CD19+), and the absolute number of each cell type per microliter of blood. The assay is a 7-color, no-wash procedure and the absolute counts are calculated from internal bead standards. The total CD45+ lymphocyte count (reported as thousand cells per microliter) and the CD4:CD8 ratio is also reported.(Hoffman RA, Kung PC, Hansen WP, Goedstien G: Simple and rapid measurement of human T lymphocytes and their subclasses in peripheral blood. Proc Natl Acad Sci USA 1980;77:4914-4917; US Department of Health and Human Services: Guidelines for performance of CD4+ T-cell determinations in persons with human immunodeficiency virus infection. MMWR 46 no. RR-2:1997, pp 1-29)